The maser (Microwave Amplification by the Stimulated Emission of Radiation) is in fact the forerunner of the laser (Light Amplification by the Stimulated Emission of Radiation), and was discovered around 50 years ago by Townes, Basov and Prokhorov who shared the 1964 Nobel Prize in Physics for this work. A laser can be thought of simply as maser that works with higher frequency photons in the visible light spectrum, whereas a maser works at microwave frequencies. Both systems rely on chemical species with an excited energy-level population being stimulated into lower energy levels, either by photons or collisions with other species. Photons are coherently emitted by the stimulated atom or molecule, in addition to the original photons that entered the system at the same frequency, meaning that a strong beam of monochromatic radiation is produced.
Although lasers appeared after masers, they are manufactured in their billions and have found their way into applications in all sectors of industry and modern-day life from DVD players to laser eye surgery. Masers, on the other hand, are used only in very specialised applications such as atomic clocks and as amplifiers in radiofrequency telescopes.
There are two key reasons why masers have not been widely adopted. The maser process only works efficiently and continuously below a temperature of ˜20K. This means that either liquid cryogens or cryo-coolers must be used. The gain of a maser amplifier typically saturates at a low signal power (typically −40 dBm). They can thus only be used as preamplifiers of weak signals and not as power amplifiers. To work at a particular frequency, a d.c. magnetic field must be applied to the maser, limiting the frequency to a few tens of MHz. Thus, a maser amplifier has sufficient bandwidth to amplify a single non-compressed video signal but little more. Compared with the GHz bandwidths offered by semiconductor-based microwave amplifiers, masers are restrictively narrow-band devices. Applying the magnetic field means using bulky electromagnets and water cooling or using superconducting magnets at cryogenic temperatures. Both are bulky and power hungry.
Research by Blank et al [1] considered the realisation of optically-pumped masers at room temperature using organic polyaromatic hydrocarbon molecules such as C60 and porphyrins, placed within strong d.c. magnetic fields. These molecules exhibit very long spin-polarization lifetimes. Importantly, the authors believed that the so-called “triplet mechanism” would not produce masing at room temperature.
An equation can be simply derived that governs maser action for continuous-wave optical pumping, as follows:
            (                                    μ            0                    ⁢                      γ            2                                    π          ⁢                                          ⁢                      c            0                              )        ·                  κ        ⁢                                  ⁢                  P          opt                ⁢        λ                    ︸        optical              ·                            T          1                ⁢                  T          2                ⁢                  η          ISC                ⁢        Δ        ⁢                                  ⁢        N                    ︸        triplet              ·                  F        m                    ︸        resonator              >  1where μ0 is the permeability of free-space, γ is the electron gyromagnetic ratio, κ is the optical coupling efficiency, Popt is the optical pumping power with wavelength λ, T1 is the triplet inversion lifetime (spin-lattice relaxation), T2 is the spin-spin relaxation time, ηISC is the intersystem crossing yield from the excited S1 state, ΔN is the difference in normalised populations of the triplet inversion, Fm is the magnetic Purcell factor (defined as Q/Vm, where Q is the Q-factor and Vm is the magnetic mode volume) and c0 is the speed of light in vacuum.
From this equation it can be seen that three factors contribute towards achieving maser action: the optical pumping system, the triplet-state molecules and the resonator Purcell factor. For a given molecular system providing an inverted population of triplet-states, the optical pumping power required to achieve threshold is inversely proportional to the magnetic Purcell factor and hence by maximising this, the optical pumping power required can be reduced.
The present work seeks to increase the magnetic Purcell factor, and thereby reduce the optical pumping power required, by virtue of the configuration of the resonator structure and/or the materials used in its construction.
Although, in the present work, the structures and materials are primarily described in relation to masers (i.e. to generate stimulated emission of microwave radiation), it is to be emphasised that they may also be used to generate stimulated emission of electromagnetic radiation at other wavelengths, in particular at radio frequencies.